1. Field of the Invention
The invention relates generally to data acquisition and interpretation of data for subterranean acidizing operations. In other aspects, the invention relates to data acquisition and interpretation for subterranean fracturing operations.
2. Description of the Related Art
Matrix acidizing is a stimulation process wherein acid is injected into a wellbore to penetrate rock pores. Matrix acidizing is a method applied for removing formation damage from pore plugging caused by mineral deposition. The acids, usually inorganic acids, such as fluoridic (HF) and or cloridic (HCl) acids, are pumped into the formation at or below the formation fracturing pressure in order to dissolve the mineral particles by chemical reactions. The acid creates high-permeability, high productivity flow channels called wormholes and bypasses the near-wellbore damage. The operation time depends on such parameters as the length of the wellbore, the rock type, severity of the damage, acid pumping rate, downhole conditions and other factors.
Matrix acidizing is also useful for stimulating both sandstone and carbonate reservoirs. Matrix acidizing efficiency in removing the formation damage is strongly dependent on the temperature at which the acidizing occurs and weakly dependent upon the corresponding pressure. The acid temperature in the formation depends on the convective heat transfer as the acid flows through the formation and on the reaction heat transfer due to the acid-mineral reaction.
Convective heat transfer is the main mechanism for temperature change during acid flow through wormholes. The acid temperature in the wormholes may vary by as much as 10-20° C. (18-36° F.), or more, depending on the initial temperature difference between wellbore and the formation. The acid temperature at the end of the wormholes, about 1-10 m (3.3-33 feet) from the wellbore, may increase, for example, by 1°-5° C. (1.8°-8° F.) above the formation temperature at those locations, depending on the injected acid volume.
Along a wormhole, the temperature changes over time as illustrated by FIG. 4. Initially, the temperature near the wellbore is the acid temperature inside the well (Tw at t=0). The rest of the wormhole, which may be partially or totally undeveloped, is assumed to be at the formation or reservoir temperature (Tr at t=0), which is greater than the wellbore temperature. As time progresses and acid is injected through the wormhole, at small radial distances near the wellbore (up to about 1 m (3.3 feet)), the acid temperature decreases from Tr to Tw with time at a rate depending upon the temperature drop of the fluid flowing from the wellbore. In other words, in the near well region, the temperature behavior depends only on the convection heat transfer due to the acid flow through the wormhole.
A diversion technique is important to success of matrix acidizing. In a diversion technique, diverter fluid is pumped into the wellbore prior to injection of acid. Optimum acid placement is also important. Stimulation efficiency depends to a great deal upon temperature. Temperature will accelerate or decelerate the chemical reactions and, thus, impact the acid volumes and flow rates required for an optimal treatment.
Prior art techniques for obtaining temperature data during acidizing have sought to provide such data in “real time.” In most cases, a distributed temperature sensing (“DTS”) fiber is inserted into coiled tubing which is then run into the wellbore. A DTS fiber is an optic fiber having sensors along its length. The acid is bullheaded, and the DTS-enabled coiled tubing is left in place within the wellbore for hours, and temperature traces along the entire stimulated interval are acquired and interpreted at surface. Although this method is marketed as being “real time” in the industry, it has two major disadvantages which hinder its effectiveness. First, the fiber is located inside the coiled tubing and does not have direct contact with the acid. Thus, its readings depend upon the heat transfer from the annulus acid through the coiled tubing wall, to the DTS fiber. Second, this is not actually a real time technique, since long periods of time, usually hours, are reported for acquiring time-dependent temperature traces. Indeed, after the acid is bullheaded, the DTS software could evaluate the temperature profile and recommend more stimulation needed in certain zones. Then a new diversion/acidizing treatment would have to be executed in order to inject more acid into the targeted zone(s).
In some instances, a DTS fiber is secured to the radial exterior of the completion. In these cases, the DTS fiber installation is permanent. But the arrangement is typically very costly to maintain and prone to failure. Additionally, it cannot be used in an open hole well that has not been completed.
In fracturing operations, a fracturing fluid, usually containing proppant, is injected into a wellbore at selected locations. At present, there is no reliable method for determining the flow rates for injected fracturing fluid at locations within the wellbore in real time.